The commercial production of olefins such as ethylene and propylene relies mainly on the pyrolysis of light hydrocarbon feeds at high temperatures. Thermal cracking of ethane, propane or higher hydrocarbons invariably leaves un-cracked paraffins and other undesirable compounds in the product stream. The undesirable paraffins (e.g. ethane, propane etc.) must be separated from ethylene, propylene and other products which, due to the similar boiling points of paraffins and olefins having the same carbon number, requires the use of energy intensive cryogenic distillation columns. Such “superfractionations” represent a significant portion of the cost associated with running a cracking unit. Specifically, it would be beneficial if expensive C2 or C3 splitter columns could be augmented or replaced.
In the interests of reducing cost and operating complexity, several methods have been explored to replace the expensive separation processes used in traditional hydrocracking plants. These include selectively adsorptive membranes (see for example U.S. Pat. Nos. 6,395,067; 6,340,433; Kotelnikov et al. in Stud. Surf. Sci. Catal. 2004, v 147, p 67 and Bryan et al. in Sep. Purif. Rev. 2004, v 33, p 157), liquid extraction systems, and pressure swing adsorption methods (see for example U.S. Pat. Nos 3,430,418; 4,589,888; and 6,497,750).
Pressure swing adsorption (PSA) processes generally include i) a high pressure adsorption step, during which a component in a gaseous mixture is selectively adsorbed onto an adsorbent substrate ii) a purging step, during which non-adsorbed components are collected as waste, recycle or product effluent; and iii) a low pressure de-sorption step or regeneration step, during which the selectively adsorbed component is released form the adsorbent substrate (see for example, U.S. Pat. No. 6,197,092 that is incorporated herein by reference). In a PSA processes, the adsorbent material is typically packed in one or more beds, and various pressurization/depressurization protocols including the application of vacuum can be used (see Adsorption, Gas Separation in the Kirk-Othmer Encyclopedia of Chemical Technology, Copyright John Wiley & Sons, Inc. vol 1, pg 617 and references cited therein).
Several types of adsorbents have been developed for the separation of various gas mixtures by PSA processes, and the useful application of each depends mainly of the nature of the gases to be separated. PSA, and similar separation processes such as thermal swing adsorption (TSA), may utilize a kinetically effected separation, which excludes one potential adsorbent due to pore diameter restrictions in the adsorbent, and/or thermodynamically effected separation, in which one potential adsorbate binds more strongly to the adsorbent than another potential adsorbate under equilibrium conditions. Thermodynamic separations may be facilitated by electrostatic or bonding interactions between an adsorbent material and an adsorbate molecule.
Adsorbents for the separation of olefins from paraffins often include high surface area, porous materials which have been treated with metal species capable of π-complexation with olefins, such as copper and silver salts. For example, U.S. Pat. No. 4,917,711 describes the use of supports such as zeolite 4A, zeolite X, zeolite Y, alumina and silica, each treated with a copper salt, to selectively remove carbon monoxide and/or olefins from a gaseous mixture containing saturated hydrocarbons (i.e. paraffins) such as ethane and propane.
U.S. Pat. Nos 6,867,166 and 6,423,881 describe the use of copper salts and silver compounds supported alternatively on silica, alumina, MCM-41 zeolite, 4A zeolite, carbon molecular sieves, polymers such as Amerberlyst-35 resin, and alumina to selectively adsorb olefins from gaseous mixtures containing olefins and paraffins. Both kinetic and thermodynamic separation behavior was observed and modeled.
Clay based adsorbents which have been treated with silver salts are taught by Choudary et al. in the Ind. Eng. Chem. Res. 2002, v 41, p 2728. The article describes Ag+ impregnated clay adsorbents that are selective for olefin uptake from a gaseous olefin/paraffin mixture. Up to 20% of the olefin is adsorbed in an irreversible manner. The adsorbent was evaluated for its performance in a four bed vacuum swing adsorption process. Ethylene was separated from ethane with over 85% recovery and in over 99% purity.
An article in Chemical Engineering Research and Design, 2006, 84(A5) p 350, by Van Miltenburg et al. reported the use of Cu+ to modify Faujasite zeolites. The modified zeolites were useful adsorbents for the separation of ethylene from ethylene/ethane mixtures. The use of similarly modified Faujasite zeolites in a highly selective PSA process that separates carbon monoxide and/or olefins from a mixture that also contained paraffins was reported in U.S. Pat. No. 4,717,398 assigned to BP.
In U.S. Pat. Nos 5,744,687; 6,200,366 and 5,365,011 assigned to BOC, copper modified 4A zeolites were used to separate ethylene and propylene form ethane and propane respectively. Elevated temperatures were required for successful application to PSA processes (i.e. from 50° C. to 200° C.). Zeolites such as zeolite 5A and zeolite 13X were also used in the formation of copper modified adsorbents.
U.S. Pat. No. 6,293,999 assigned to UOP, describes the use of aluminophosphates to separate propylene from propane in a PSA process. The aluminophosphate is a small pore molecular sieve designated “AIPO-14”. The system operates at temperatures of from 25° C. to 125° C. to effect a kinetic separation of propylene from propane. U.S. Pat. No. 6,296,688 also to UOP, discloses a vacuum swing adsorption process for separating propylene form propylene/propane mixtures using analogous zeolite adsorbents.
Despite the above progress, new materials having high selectively and good pressure swing capacity are still needed for olefin/paraffin separation processes. Particularly desirable are adsorbents that can be tuned to suit commercial process conditions or adsorbents that are effective in ambient temperature PSA separation of olefin/paraffin mixtures.
In U.S. Pat. Nos 4,938,939 and 5,011,591, assigned to Engelhard Corp., a new family of crystalline titanium silicate zeolite materials was disclosed.
U.S. Pat. No. 4,938,939, describes a small pore zeolite, designated “ETS-4” with pore diameters of about 3-5 Å. Modification of the ETS-4 materials by cation exchange with for example, Ba2+ and Sr3+ gave adsorbents which were useful in the separation of nitrogen from methane using PSA processes (see U.S. Pat. Nos 6,068,682 and 5,989,316).
As described in U.S. Pat. No. 6,517,611, heat treatment of ETS-4 gave a controlled pore volume zeolite material, dubbed “CTS-1” which is a highly selective absorbent for olefin/paraffin separations. The CTS-1 zeolite, which has pore diameters of from about 3-4 Å, selectively adsorbed ethylene from a mixture of ethylene and ethane through a size exclusion process. The pore diameter of CTS-1, allowed diffusion of ethylene, while blocking diffusion of ethane which was too large to enter the pores of the CTS-1 zeolite, thereby providing a kinetic separation. The CTS-1 adsorbent was successfully applied to a PSA process in which ethylene or propylene could be separated from ethane or propane respectively.
U.S. Pat. No. 5,011,591 discloses the synthesis of a large pore diameter titanosilicate designated “ETS-10”. In contrast to ETS-4 and CTS-1, the large pore titanosilicate material, ETS-10, which has pore diameters of about 8 Å, cannot kinetically distinguish light olefins from paraffins of the same carbon number. Nevertheless, high degrees of selectivity have been reported for the separation of ethylene from ethane using as prepared ETS-10 zeolites; see: Al-Baghli and Loughlin in J. Chem. Eng. Data 2006, v 51, p 248. The authors demonstrate that Na-ETS-10 is capable of selectively adsorbing ethylene from a mixture of ethylene and ethane under thermodynamic conditions, even at ambient temperature. Although, the reported selectivity for ethylene adsorption using Na-ETS-10 was high at ambient temperature, the adsorption isotherms for ethylene and ethane had highly rectangular shapes consistent with a low pressure swing capacity. Consequently, Na-ETS-10 is not readily applicable to pressure swing absorption processes (PSA), at least at lower or ambient temperatures.
We have now found that the separation selectivity and pressure swing capacity of Na-ETS-10 can be dramatically affected by cation exchange. The resulting modified ETS-10 zeolites provide more useful pressure swing capacities for olefin/paraffin separation. In addition, the modified ETS-10 zeolites can be precisely tuned by cationic exchange to cover a range of adsorbent behavior from silica type adsorbents (i.e. weak adsorbents) to more traditional zeolites (i.e. strong adsorbents). Hence, the ETS-10 zeolites can be modified to suit a wide range of PSA process conditions for the separation of olefins from paraffins and in some cases are suitable for ambient temperature PSA.